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From Shawn Heisey <apa...@elyograg.org>
Subject Re: solrcloud load balancing
Date Sun, 23 Oct 2016 01:14:27 GMT
On 10/22/2016 6:19 PM, Jay Potharaju wrote:
> I am trying to understand how load balancing works in solrcloud.
>
> As per my understanding solrcloud provides load balancing when querying
> using an http endpoint.  When a query is sent to any of the nodes , solr
> will intelligently decide which server can fulfill the request and will be
> processed by one of the nodes in the cluster.

Erick already responded, but I had this mostly written before I saw his
response.  I decided to send it anyway.

> 1) Does the logic change when there is only 1 shard vs multiple shards?

The way I understand it, each shard is independently load balanced.  You
might have a situation where one shard has more replicas than another
shard, and I believe in that even in that situation, all replicas should
be used.

> 2) Does the QTime displayed is sum of processing time for the query request + latency(if
processed by another node) + time to decide which node will process the request(which i am
guessing is minimal and can be ignored)

There are three phases in a distributed (multi-shard) query.

1) Each shard is sent the query, with the field list set to include the
score, the unique key field, and if there is a sort parameter, whichever
fields are used for sorting.  These requests happen in parallel. 
Whichever request takes the longest will determine the total time for
this phase.

2) The responses from the subqueries are combined to determine which
documents will make up the final result.

3) Additional queries are sent to the individual shards to retrieve the
matching documents.  These requests are also in parallel, so the slowest
such request will determine the time for this whole phase.

> 3) In my solr logs i display the "slow" queries, is the qtime displayed
> takes all of the above and shows the correct time taken.

For non-distributed queries, QTime includes the time required to process
the query, but not the time to retrieve the documents and send the
response.  I *think* that when the query is distributed, QTime will be
the sum of the first two phases that I mentioned above, but I'm not 100%
sure.

Thanks,
Shawn


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